Electrical measuring instrument



Aug. 7, 1945.- i D. A. YOUNG ET AL I I ELECTRICAL MEASURING INSTRUMENT Filed Dec. 12, 1942 2 Sheets-Sheet 1 lNVE-NTORS D. A. YOLTING ET AL 2,381,329

Aug. 7,v 1945.

EIJECTRICAL MEASURING INSTRUMENT Filed Dec. 12, 1942 2 sheetsf-sheet 2 35 ATTORNEY measuring instrument assembly. The panel member I I also includes four openings I5 through which screws may be passed into threaded engagement within the threaded openings 5 of the cover I. Such screws H, (see Fig. 2) are applied from the rear of the panel member II to secure firmly the cover I to the panel member.

When the panel member II is designed to receive an electrical measuring instrument, a plurality of terminals or binding posts may be mounted thereon to facilitate connections between the instrument movement and an. electrical circuit. As a specific example, the panel member II may be designed to receive a combined voltmeter and ammeter. In such a case, one of the terminals I9 may be designated as a common terminal to which one lead from the associated electrical circuit is always connected. The remaining lead from the electrical circuit may be connected to one of the terminals 2|, 23, or 21 for the purpose of measuring current flowing in the electrical circuit. Each of. the latter terminals provides a different current range for the associated electrical instrument movement.

If the voltage of the associated electrical circuit is to be measured, one lead from the electrical circuit is connected to the terminal I9 and the remaining lead is connected to one of the terminals 29, 3| or 33. Each of the latter terminals provides a different voltage range for the associated electrical instrument movement. If desired, the voltage terminals 29, 3|, 33 normally maybe out of circuit and may be connected in circuitb-y means of a switch 35 having an operating push button 33 which projects through the panel member II. This push button normally is biased outwardly to connect the current terminals 2 I, 23, 25 and 21 to the associated measuring instrument movement. When the push button 36 is depressed, the connection between the terminals 2|, 23, 25 and 21, the instrument movement is broken and a connection is established between the instrument movement and the terminals 29, 3| and 33 for the purpose of measuring voltages. This terminal and switch arrangement is well known in the art. If desired, the terminals may be of the type disclosed in the copending application, Serial No. 454,930, filed August 14, 1942, of V. S. Thomander, which is assigned to the Westinghouse Electric & Manufacturing Company.

After the cover I has been attached to the panel member I I an instrument movement 31 may be inserted within the shield 3. The instrument movement 31 may be of any desired type, such as a moving-iron instrument movement. For the purpose of discussion, it will be assumed that the instrument movement 31 is a permanent-magnet moving coil instrument having a dial 39 across which a pointer 42 is movable. The instrument is attached to a base plate 4| by means of a pair of posts 43 which may be secured to the base plate 4| by means of suitable nuts 45 which are in threaded engagement with the posts 43 (see Fig. 3). In addition, a third post 41 may be attached to the base plate 4| in any suitable manneras by a riveting operation and other portions of the instrument movement may be secured to the'post 4'I. As shown more clearly in Fig. 2, the base plate 4| is secured to the panel member II by means of machine screws 49 passing through openings 50 provided'in the base plate 4| and secured in threaded openings provided in the panel member I Preferably spring washers 5| are employed with the effected through a connecting lug 63.

machine screws 49 for providing a resilient engagement between the base plate 4| and the shield 3. In order to protect fully the instrument movement positioned within the shield 3, the shield 3 and the base plate 4| have a dustproof engagement such as that provided by grinding the engaged surfaces of the shield 3 on the base plate 4| into accurate plane surfaces. If ground surfaces are provided on these members, a gasket is unnecessary to seal the joint therebetween.

The open front instrument assembly is completed by a base member 53 having a recess for receiving the auxiliaryequipment and portions of the shield 3 and base plate 4| which project from the panel member II. This base member 53 is secured to the panel member II by means of machine screws 55 which are received in threaded openings 51 provided in the panel member II. These threaded openings are shown in Fi 3.

A typical example of connections suitable for a measuring instrument of the type herein discussed is illustrated in Fig. 3. vAs shown in Fig. 3, the instrument movement has its moving coil connected at one end to a terminal 59 which passes through an insulating and sealing bushing 6| positioned in the base plate 4|. The remaining terminal of the moving coil is grounded to the base plate 4| and connections thereto may be In the specific embodiment of Fig. 3, the connecting lug 63 is connected through a switch blade 65 to the switch 35. The normally closed contact of the switch 35 is connected'to the terminal 21 through an insulated conductor 61 anda suitable resistor 69. It will be observed that a series of graded shunts II, I3, I5 and II are connected between appropriate pairs of the current terminals. These'shunts are so graded that the range of the instrument movement increases as the connection of the adjustable lead from the associated electrical circuit changes from the terminal 2! to the terminals 25, 23, and 2|.

The normally open contact of the switch 35 is connected through conductors I9 and 8| and suitable resistors 83, 85, 81, 89, 9| and 93 to the terminals 29, 3| and 33. When the push button 36 of the switch 35 is depressed and the leads from an associated electrical circuit are connected between the terminal I9 and one of the terminals ment 31 and base member 53 again are employed.

However, the cover I of Fig. 1 is replaced by a cover IIII which is illustrated in Figs. 4, 5 and 6. The cover' IiII is similar in general to the cover I of Fig. 1. It includes the shield 3 for surrounding the instrument movement and the lightpermeable window 2 for exposing the pointer and dial of the instrument. It will be noted that the cover IIII has a contour designed to fit the contour of the panel member I I'.

A suitable method for assembling the window '2 and the shield 3 in the associated cover is shown in Fig. 5. Referring to Fig.5, it will be observed that the window 2 engages a flanged portion I03 net moving-coil instruments, it is assumed that of the. cover lfll. Conveniently thewindow 2 may terminate short of the adjusting element I: To assure a dust-proof connection between the window 2 andthe cover IOI a suitable sealing medium such as putty, may be employed between the window andthe cover. In addition, a resilient locking ring I05 having an intimate engagement with the shield 3 may be forced into the shield against the window 2 to secure the window firmly in mounted position. The adjusting element 1 may take theform of a headed pin having a resilient conicalwasher or looking ring, I 01 intimately engaging its inner end to retain the adjusting element on the cover I0 I} For protecting the window 2 and the terminals mounted on the panelII a cap I09 is attached to the cover IN by means of ahinge I I I. The cover IOI is provided-with a notch definin a flange II3 to which one portion of the hinge III is attached by means of bolts I I5. The cap I09 includes a flange II! to which thehinge .I II is attached bymeans of bolts' I I9. As shown in Fig.

5, the cap I09 in closed position completes a closure for-the window 2 and the terminalsmounted onthe panel member II. In, the open position illustrated in Fig, 6, the cap I09 permits free access to the terminals and the window. The cap I09 may be retained in closed position by means of. pins I2I positioned in the cover WI and urged outwardly against stops by means of springs (not shown). These pins are received in cups or depressions I23 formed in the cover I09 when the cover is in closed position. The construction of such pins is well'understood in the art. The

cover IOI includes threaded openings I25 (see 6) which-correspond to the openings 5.pro vided in the cover I of Fig. 1. It will be recalled that these threaded openings facilitate the mounting-of the cover on the panel member II.

To simplify the drawings, it is assumed that the instrument assembly of Fig. -5.does not require the push button switch 31 or the terminal 33 of Fig. LA. Therefore, the cover II is not provided with recesses for exposin the push button and terminal. Also the connections for the instrument movement 31 are not shown in Fig. 5.

The materials employed for the instrument assemblies may vary'appreciably. Conveniently, the base member 53, the panel member II, the cover I or IM and the capIUB may be formed the shield 3 and the base plate II are formed of magnetically soft iron or steel. It should be observed that such material also affords some protection against alternating electrical fields. To insulate the instrument movement 31 magnetical- 1y from its container, the posts 43 and 41 may be formed of a non-magnetic material such as brass.

, It is believed that the interchangeability of the covers I and I0I is apparent from the foregoing description thereof. the cover panel by detaching the machine screws I! therefrom and the cover IN is positioned inits place on the panel. When the screws I! are actuated to attach the'cover ml to the panel, the shield 3 carried by the cover IOI surrounds the instrument movement 31 to protect the instrument I movement. Such a cover replacement may be effected either at the factory or in the field.

Although the invention has been described with 7 reference to certain specific embodiments thereof, numerous modifications are possible. Therefore, the invention is to be restricted only by the appended claims.

We claim as our invention:

1. In an electroresponsive measuring instrument assembly, a casing including a base member having a recess therein, and a panel member for closing the recess in. said base member, means operable externally of said casing for attaching said panelmember to saidbase membensaid panel member having an opening therein permitting access to said recess, a cover member for said opening, said cover member including a light-permeable member for exposing'a portion of an instrument ositioned in said recess, and a shield member positioned to project from said cover member through said opening into said recess to surround an instrument positioned in said casing, said shieldmember-having an opening displaced from said cover member to permit insertion of an instrument in said shield-member while said shield member is attached to saidcover member, a closure member for said shield memof an insulatingmaterial such as a phenolic-resin.

Such material may be molded readily to anydesiredcontour.

If the shield 3 and the base plate llv are .de-

signed solely to protect the instrument'movement 31 from exposure. they may be formed of an suitable, material. -However, it is customary to provide electrical instrument movements with shielding 'againstelectrical and magnetic fields. To shield the instrument movement 31 against an alternating field,- the shield 3 and base plate AI maybe formed of an electroconductive material such as brass or copper. Eddycurrents in.- duced in an electroconductive material of this type effectively protect the instrument movement from alternating fields. As a protection against continuous magnetic fields,,,the shield 3 and the base plate 4| may be formed of a magnetic materialhaving low residual induction, such as magnetically soft iron or. steel. 'It will be observed that the shield 3 includes a portion 3A extending over a portion of the front wall of the cover IM to increase the electrical 'shieldingaction of the shield 3. Since magnetic shielding is desirable in electrical instruments, such as permanent-magber, said closure -member, shield member and light-permeable member forming a substantially dust-proof container extending through the opening in said panel member into said recess, an electroresponsive instrument positioned with,- in said container for exposure through said lightpermeable member, and means attaching said instrument to said closure member for removal therewith relative to said cover member, said shield member being effective for shielding said instrument against fields produced by. electric charges. I

2\. In a measuring instrument assembly, an electrical measuring instrument, 'a base plate, means securing said measuring instrument to said base plate, a magnetic shield member surrounding said measuring instrument, said shield member having a first open end engaging said base plate with asubstantially dust-proof fit To replace the cover I by IOI, the cover I is removed from its member for securing said cover means to said panel member, means securing said base plate to said panel member and said shield member, auxiliary means supported on the rear face of said panel member for modifying the range of response of said instrument movement, terminal means accessible from the front of said panel member for connecting said instrument movement to an electrical circuit, and a casing having a recess closed by said panel member, said casing and said panel member forming a closure for said auxiliary means and the portionof said container in the rear of said panel member.

3. In a measuring instrument assembly, an electrical measuring instrument, a, base plate, means securing said measuring instrument to said base plate, a magnetic shield member surrounding said measuring instrument, said shield member having a first open end engaging said base plate with a substantially dust-proof fit therebetween, said shield member having a second open end for exposing a portion of said instrument, a light-permeable member positioned adjacent said second open end, cover means engaging said shield member ,and said light-permeable member for completing therewith a substantially dust-proof container for said instrument, a panel member having an opening receiving said shield member, said panel member having a front face engaging said cover means, resilient means operable from the rear of said panel member for securing said cover means to said panel member, means securing said base plate to said panel member and said shield member, auxiliary means supported on the rear face of said panel member for modifying the range of response of said instrument, terminal means accessible from the front of said panel member for connecting said instrument to an electrical circuit, a casing having a recess closed by said panel member, said casing and said panel j member forming a closure for said auxiliary means and the portion of said container in the rear of said panel member, and a cap member pivoted to said cover member for movement from a position, enclosing to a position exposing said terminal means and said light-permeable member.

4. In a housing for electroresponsive measuring instruments, a casing including a base member having a recess therein, and a panel member for closing the recess in said base member, means operable externally of said casing for'attaching said panel member to said base member, said panel member having an opening therein permitting access to said recess, a cover member for said opening, said cover member including a light-permeable member for exposing a portion of an instrument positioned in said recess, and a shield member positioned to project from said cover member through said opening into said recess to surround an instrument positioned in said casing, said shield member and said lightpermeable member cooperating to provide a substantially dust-proof protective closure for an instrument positioned in said recess, said shield member being formed of a material suitable for shielding an instrument positioned therein from external electromagnetic fields, and said shield member having an opening displaced from said cover member, whereby an instrument may be inserted through said opening intoproximity to said light-permeable member without disturbing the relationship between said shield member and said cover member.

means operable from the rear of said panel 5. In an electroresponsive measuring instrirment assembly, a casing including a base member having a recess therein, and a panel member for closing the recess in said base member, means operable externally of said casing for attaching said panel member to said base member, said panel member having an opening therein permitting access to said recess, a cover member for said opening, said 'cover member including a light-permeable member for exposing a portion of an instrument positioned in said recess, and a shield member positioned to project from said cover member through said opening into said recessto surround an instrument movement positioned in said casing, said shield member having an'opening displaced from said cover member ,to permit insertion of an instrument in said shield member while said shield member is attached to said cover member, a detachable closure member for said shield member, said closure member, shield member and light-permeable member forming a substantially dust-proof container extending through the opening in said panel member into said recess, an electroresponsive instrument positioned within said container for exposure through said light-permeable member, and means attaching said instrument to said closure member for removal therewith relative to said cover member, said shield member and said closure member being formed of a material suitable for shielding said instrument from electromagnetic fields.

6. In an electroresponsive measuring instrument housing, a casing including a. base member having a recess therein, and a panel member for closing the recess in the base member, said panel member having an opening therein permitting access to said recess, a, cover member for said recess, means detachably securing said cover member to the base member, said cover member including a light-permeable member for exposing a portion of an instrument positioned in the easing, and a shield member projectingfrom the cpver member into the recess for substantially surrounding and shielding from fields produced by electric charges an, instrument positioned in the casing, said shield member being secured in substantially fixed position to the cover member for removal as'a unit therewith from said panel member.

'7. In an electroresponsive measuring instrument housing, a casing including a base member having a recess therein, and a panel member for closing the recess in the base member, said panel member having an opening therein permitting access to said recess, a cover member for said recess, means detachably securing said cover member to the base member, said cover member including a light-permeable member for exposing a portion of an instrument positioned in the casing, a tubular shield member projecting from the cover member into the recess for substan tially-su'rrounding and shielding from fields produced'b'y electric charges an instrument positioned in the casing, said shield member being permanently secured with a substantially dustproof union to the cover member for removal as a unit therewith from the panel member, and a closure member for said tubular shield member, said cover member, shield member and closure member forming a container for an instrument.

DOUGLASS A. YOUNG.

l l E VERON S. THOMANDER. 

